1. Field of the Invention
The present invention is related to an amplifier device capable of reducing offset voltage, and more particularly, to an amplifier device that completely cancels offset voltage through a pseudo output stage circuit.
2. Description of the Prior Art
An operational amplifier is an important component of all kinds of electronic devices, and is widely used in home appliances, industrial fields, scientific instruments, etc. Circuit designers usually use operational amplifiers for different operations, such as buffers, filters, analog to digital converters, etc.
An ideal operational amplifier has the following characters: infinite input impedance, zero output impedance, infinite open loop gain, infinite common mode rejection ratio, infinite frequency bandwidth. However, due to limitations of semiconductor processes and integrated circuit techniques, there are difficulties in realizing an ideal operational amplifier. In order to present properties of a real operational amplifier, an offset voltage is used for representing the non-ideal effects in the prior art. In the current technology, many offset voltage cancellation methods and related devices are presented.
For example, please refer to FIG. 1, which illustrates a schematic diagram of a prior art amplifier device 100 capable of reducing offset voltage. The amplifier device 100 comprises an operational amplifier 110, an offset voltage cancellation device 120, and an equivalent voltage source 102. The gain of the operational amplifier 110 is 1, referring that an output voltage Vo equals an input voltage Vi. The operational amplifier 110 comprises an input stage circuit 1101 and an output stage circuit 1102. The equivalent voltage source 102 (which does not exist on an actual circuit) represents the offset voltage Vos that is generated by the non-ideal effect of the operational amplifier 110. The offset voltage cancellation device 120 is utilized for compensating the offset voltage, which comprises switches S11, S12, S21, and a capacitor Cs.
The offset voltage cancellation device 120 reduces the offset voltage Vos effects with a virtual short characteristic of the operational amplifier 110. Firstly, when the amplifier device 100 operates in an offset voltage storing mode, the switches S11 and S12 are turned on, and the switch S21 is turned off, so that the voltage difference (which is the offset voltage Vos) between a positive input end and an output end of the operational amplifier 110 charges the capacitor Cs. Then, when the amplifier device 100 operates in an offset voltage cancellation mode, the switch S21 is turned on, and the switches S11 and S12 are turned off. The amount and the polarity of the voltage difference stored between the two ends of the capacitor Cs can compensate the effect of the offset voltage Vos, to make the voltage difference between the positive end and the output end Vo become 0, so as to reduce the offset voltage Vos.
When the output voltage Vo of the amplifier device 100 changes with the input voltage Vi, the offset voltage cancellation device 120 can compensate the effect of the offset voltage Vos through turning switches S11, S12, S21 on and off. However, when the output end of the operational amplifier 110 is coupled to a capacitor, the duration when the voltage Vo changes with the input voltage Vi is affected by the slew rate of the operational amplifier 110, which means that the output voltage Vo does not change with the input voltage Vi synchronously, so that the voltage difference between the output voltage Vo and the input voltage Vi does not equal 0 during a certain duration. Under this condition, the voltage stored in the capacitor Cs transiently changes with the offset of the output voltage Vo, which does not equal to the dc offset voltage Vos. As a result, when the amplifier device 100 operates in the offset voltage cancellation mode, the offset voltage Vos can not be completely reduced and affects performances and the application range of the amplifier device 100.